All ignition systems need to know at what position the engine is in its cycle to be able to perform accurate spark timing calculations. This information is generally provided by a timing disc attached either to the crankshaft (engine speed) or camshaft (½ engine speed) and an electronic sensor mounted close by. Timing discs either have teeth or magnets arrayed around the circumference, which the electronic sensor can ‘see'. To enable the igniter to know when the first cylinder is at top dead centre, the timing disk either has an additional tooth or magnet or one missing.
If the timing disk is attached to the crankshaft, there is a need in some engine configurations to have a sensor on the camshaft so that the igniter knows which ½ of the four-stroke cycle the engine is in
